The invention relates to a process for reducing the size of pores and/or making catalytically active a membrane top layer in a ceramic ultrafiltration membrane system consisting of a sub-microporous top layer and a microporous inorganic carrier. An ultrafiltration membrane is understood to be a membrane with pores smaller than 0.02 .mu.m. Ceramic ultrafiltration membrane systems consisting of a microporous top layer and a microporous inorganic carrier, as well as a process for the production thereof, are known from EP-A-0144097.
Gas separation applications (on the basis of Knudsen diffusion or other mechanisms) requires a high quality top layer and the top layer on the carrier according to EP-A-0144097 often does not meet these quality requirements. This requirement is that 100% Knudsen diffusion (within a measuring error of 2%) takes place for helium in the top layer up to a pressure of 10 bar. If this is not the case, the procedure according to EP-A-0144097 is repeated once or twice (repair). The selectivity of gas separation according to the Knudsen diffusion mechanism is low (equal to the root of the mol mass ratio of the gases) and can be improved by reducing the pore size via modification in such membrane systems.
This is necessary for realizing new applications for such membrane system, e.g., in oil conversion processes. Another example of modification is the application of a catalytically active material onto and/or into the membrane top layer and the use as catalytically active membrane system in chemical processes at higher temperatures (&gt;200.degree. C.).
The invention now provides a process and a membrane with which these objects can be achieved.